The present invention relates to a method and apparatus for installing a pile and/or a concrete or grout column in the ground, in particular by way of soil displacement.
It is known to install load-bearing piles or columns by various methods. A first method involves hammering a preformed pile into the ground in a series of discrete steps. This method can be effective, but there is a risk of causing damage to the pile or to the ground due to the discontinuous hammering action. Furthermore, much noise and vibration is caused by hammering. An alternative method is to use a jack to install a pile or column made up of a number of discrete sections. A first section is pushed into the ground by the jack, which is then reset, and a second section is then welded or bonded to the top of the first section. The jack is then activated again, and the process repeated until the required depth has been reached. This method is low in efficiency since the jack needs to be retracted after completing each single stroke so that the next element of the pile can be inserted, particularly since a typical stroke length is less than 50 cm.
A second known method is continuous flight auger piling, in which an auger with a continuous flight is caused to enter the ground by way of rotation. Soil is excavated by way of the auger flights before or during the time that the auger is withdrawn from the ground. As the auger is being withdrawn, concrete is pumped through the stem of the auger to the tip, thereby leading to the formation of a load-bearing pile or column. Such a method is described in the present applicant""s U.K. patent application no. 9515652.7, the disclosure of which is incorporated into the present application by reference.
Alternatively, as disclosed for example in WO 95/12050, it is possible to use an auger head which does not excavate soil, but instead displaces the soil and compacts it into the surrounding ground. This has the advantage that less spoil is generated, and can lead to better maintenance of ground integrity and greater density in the vicinity of the pile installation.
However, both these methods require that an auger or similar device be screwed into the ground, which takes a relatively long time and generally means that specific combinations of torque and thrust must be applied in accordance with the ground conditions in order to achieve penetration, and these can be difficult to achieve. Another disadvantage with these screw piling methods is that the piling tool is subjected to a large degree of wear. Furthermore, we have found that there appears to be an inverse relationship between the rotational torque required to produce downward thrust by way of the pitch of the auger flights and the xe2x80x9ccrowd forcexe2x80x9d (i.e. the total force applied along the longitudinal axis of the auger during penetration) generated at the rig which can be used to achieve penetration into the ground. Indeed, the crowd force in itself is not sufficient to achieve the desired penetration, particularly when premature resistance to penetration, e.g. caused by the presence of a subterranean stratum of granular material such as gravel, is experienced.
According to a first aspect of the present invention, there is provided a method of installing a load-bearing pile or column in the around, wherein:
i) a hole-forming tool or pile having a longitudinal axis is pushed into the ground, substantially in the direction of its longitudinal axis, in a substantially non-percussive manner to a first depth; and
ii) the hole forming tool or pile is then pushed further into the ground, substantially in the direction of its longitudinal axis, in a substantially non-percussive manner to a second depth while being rotated about its longitudinal axis.
The term xe2x80x9cnon-percussivexe2x80x9d is to be understood as the application of a substantially continuous force over a relatively protracted period of time, e.g. over a number of seconds or even minutes. This is to be distinguished from percussive methods of piling, in which, for example, a weight is repeatedly dropped onto the top of a pile so as, in effect, to hammer the pile into the ground. In this case, most of the force is applied over a relatively short period of time, e.g. fractions of a second. Furthermore, the rate of change of applied force with respect to time will, in practical terms, generally have a discontinuity, as opposed to non-percussively applied force, which will tend to have a rate of change with respect to time which is substantially continuous.
The present invention is of particular use when installing or forming piles in soft ground overlying a granular stratum, such as gravel or the like. The rotational motion of the hole-forming tool or pile helps to overcome premature resistance to penetration which may otherwise prevent the attainment of desired depths. Generally, the hole-forming tool or pile is pushed into the soft overlying soil until it reaches the granular stratum, at which point the hole-forming tool or pile is additionally rotated. Rotation combined with the pushing force is surprisingly effective in penetrating granular strata, especially with certain hole-forming tool or pile tip geometries, and enables the resulting pile to be well-founded.
The rotation may be continuous rotation in either direction; alternatively or in addition, back and forth rotation may be used, which rotation may be less than or more than one revolution. Where back and forth rotation is used, it has been found that an oscillation frequency of around 1 Hz is effective in aiding penetration of granular strata, although frequencies of an order of magnitude higher or lower, e.g. around 10 Hz to 0.1 Hz, are also envisaged. In some applications, even higher or lower frequencies, e.g. around 100 Hz to 0.01 Hz, may be useful.
This method may be used either to install a pile, such as a steel or pre-cast concrete pile, directly into the ground, or may be used to insert a hole-forming tool, such as a hollow cylindrical tube provided with a sacrificial end plate, into the ground so as to allow a load-bearing concrete or grout column to be cast-in-situ prior to or during removal of the tool. The pile or tool dimensions and the force applied to the pile or tool are advantageously determined in accordance with the ground conditions.
Preferably, the hole-forming tool or pile is pushed in a continuous motion to a given depth of at least 1 m into the ground, and in some applications, at least 2 m or even 5 m into the ground. Once the given depth has been reached, the continuous force may be reapplied one or more times so as to push the hole-forming tool or pile to a greater given depth, such as the depth of a granular stratum.
In this way, we have found that it is possible in certain ground conditions to attain depths in excess of 5 m in a time of around 16 seconds, as opposed to 4 minutes using a rotating auger.
According to a second aspect of the present invention, there is provided a method of boring into the ground, wherein:
i) a hole-forming tool having a longitudinal axis is pushed into the ground, substantially in the direction of its longitudinal axis, in a substantially non-percussive manner to a first depth; and
ii.) the hole-forming tool is then pushed further into the ground, substantially in the direction of its longitudinal axis, in a substantially non-percussive manner to a second depth while being rotated about its longitudinal axis.
The hole-forming tool is generally withdrawn from the ground after it has reached the required depth, although in some applications the tool may be sacrificed and left in the ground.
In some embodiments, the hole-forming tool has a generally pointed tip, since this can be effective in penetrating granular strata when pushed into the ground with concomitant rotation. However, a particularly preferred embodiment utilises a hole-forming tool with a generally flat base. We have found that upon pushing such a flat-based tool into the ground, soil under the flat base tends to be compressed into a cone of relatively higher density than the surrounding soil. This cone of relatively high density soil substantially stays with the base of the hole-forming tool during penetration and rotation, and serves to disturb the underlying soil in the desired manner. In some applications, it is advantageous to spread a carpet of gravel or other granular material on the top of the ground prior to penetration of the hole-forming tool. The flat-based hole-forming tool is then lowered onto the carpet of gravel or other granular material and pushed into the ground, thereby using the gravel or granular material to make up at least some of the resulting cone. Although a proportion of the soil and/or gravel or other granular material forming the cone of relatively high density may be lost to the surrounding soil, this proportion is usually relatively low, and in any case will generally be replaced by soil underlying the base of the hole-forming tool.
A pushing force is applied to the hole-forming tool in a similar manner to that described in relation to the other aspects of the present invention, as is the rotation. Advantageously, the magnitude and duration of the applied non-percussive force, as well as those of the torque applied to generate rotation, are monitored and controlled by electronic computer means.
According to a third aspect of the present invention, there is provided a rig for inserting a hole-forming tool or pile having a longitudinal axis into the ground, wherein the rig is provided with first means for applying a substantially non-percussive force to the hole-forming tool or pile so as to push this into the ground, substantially in the direction of its longitudinal axis, to a first depth, and wherein the rig is further provided with second means for applying a rotation to the hole-forming tool or pile about its longitudinal axis, so as, in combination with said first means, to push the hole-forming tool or pile, substantially in the direction of its longitudinal axis, in a substantially non-percussive manner to a second depth.
Preferably, the rig is adapted to push the hole-forming tool or pile in a continuous motion to a given depth of at least 1 m, and in some applications, at least 2 m or even 5 m into the ground. Once the given depth has been reached, the continuous force may be reapplied one or more times so as to push the hole-forming tool or pile to a greater given depth, e.g. that of a granular stratum, before the hole-forming tool or pile is rotated.
One feature of the present invention is that the substantially non-percussive pushing force applied directly to the hole-forming tool or pile is greater than the downwards forces which may be generated by rotation of the hole-forming tool or pile. This is in complete contrast to screw piling methods, in which a substantial downwards force is generated due to the reaction between the helical rotating auger flight and the soil, especially in dense, cohesive soils. Advantageously, the directly-applied force is at least two, and preferably at least five times greater in magnitude than the incidental downwards force which may be generated by rotation.
By way of the present invention, it is possible to make use of a good proportion of the weight of a typical piling rig, for example 70 tonnes, to help push. a hole-forming tool or pile into the ground. Given no constraints on the piling rig abilities, we have found that it is relatively straightforward to generate substantially only a downward force to achieve penetration of the hole-forming tool or pile. The limitations to this process are determined by the forces presented as resistance to penetration, and several fundamental factors need to be considered:
i) The resistance to penetration into the ground is dependent on the specific ground conditions and soil type.
ii) the resistance is proportional to the cross-sectional area of the element being inserted, and therefore the smaller the cross section, the lower the downward force required.
iii) The maximum downward force cannot exceed that available from the piling rig used to install the hole-forming tool or pile.
An advantage of the present invention is that once the skin friction is overcome upon penetration, the resistance experienced by the hole-forming tool or pile during motion is often less than the static resistance which may be encountered when the hole-forming tool or pile has been left undisturbed for a period during which soil xe2x80x9cset-upxe2x80x9d occurs.
It may sometimes be advantageous to apply vibration to the hole-forming tool or pile so as to reduce skin friction in the event that penetration is interrupted, but in any case the downward forces generated by such vibration will be significantly less than those required to insert the hole-forming tool or pile to a greater depth. Vibration may also be used as an aid to the expeller head should any obstructions impede its penetration into the ground.
The non-percussive force applied by the rig to the top of the hole-forming tool or pile may be provided by way of a weight adjustably suspended thereover. Alternatively or in addition, the rig may be provided with a winch arrangement which is adapted to pull down the top of the hole-forming tool or pile.
In a particularly preferred embodiment, the rig is provided with a hydraulic ram with an extension of at least 1 m, and preferably at least 2 m or in some embodiments at least 5 m. Such a ram may be used to apply a non-percussive downwards force to the top of the hole-forming tool or pile so as to achieve depths of, respectively, at least 1 m, 2 m or 5 m in a single operation. The ram may then be reset and arranged to reapply the non-percussive force so as to achieve even greater depths. Additionally, the ram may be used in conjunction with a suspended weight and/or a winch arrangement and/or a vibrator.
Rotations may be applied by way of an electric, hydraulic or pneumatic motor, or by any other suitable means, including manual means.
Where a hole-forming tool is pushed into the ground, concrete or grout may be pumped from the surface so as to emerge at; or near the tip of the tool as the tool is rotated and/or withdrawn. In this way, a cast-in-situ pile is formed. Advantageously, the volume of concrete or grout being pumped along the length of the hole-forming tool is monitored, for example by way of an electromagnetic flowmeter, and controlled by flow control means and electronic computer means. The electronic computer means additionally monitors and controls the rotation and/or the extraction of the hole-forming tool. In this way, it is possible to control the rate of rotation and/or withdrawal of the hole-forming tool as a function of concrete or grout flow, or vice versa, so as to help ensure that the resulting cast-in-situ pile is structurally sound.
According to a fourth aspect of the present invention, there is provided a method of forming an underground cast-in-situ pile, wherein:
i) a hole-forming tool comprising a body having a longitudinal axis and a lower end to which is fitted at least one blade-like attachment which extends beyond the diameter of the body is pushed into the ground, substantially in the direction of its longitudinal axis, in a substantially non-percussive manner to a first depth;
ii) at least the lower end of the hole-forming tool on which the at least one blade-like attachment is mounted is rotated in such a way that the at least one blade-like attachment displaces soil at or close to the base of the hole-forming tool while concrete or grout is concomitantly pumped along the length of the hole-forming tool so as to assist the blade-like attachments in the displacement of soil and to generate a base for the resulting pile; and
iii) the hole-forming tool is withdrawn while concrete or grout continues to be pumped along the length of the hole-forming tool.
Preferably, the hole-forming tool is pushed in a continuous motion to a given depth of at least 1 m into the ground, and in some applications, at least 2 m or even 5 m into the ground. Once the given depth has been reached, the continuous force may be reapplied one or more times so as to push the hole-forming tool to a greater given depth.
The hole-forming tool used in this aspect of the invention is particularly suited to soft ground conditions, such as soft clay overlying sands, silts overlying gravels and any generally soft material overlying granular material or suitable rock or bedrock, and is used to install a pile which can transfer load from ground level to stiffer soils at depth.
The hole-forming tool is pushed into the ground until its base reaches the required depth, which will typically be the top of a loose to medium dense sand or granular layer or rock or bedrock. A vibrator may be applied to the top of the hole-forming tool in order to aid penetration through any sand lenses or stiff layers or the like which may be encountered before the tip of the hole-forming tool reaches the required founding level. The vibrator power may be of the order of 15 bhp, although any suitable power rating may be chosen in accordance with ground conditions. It is to be noted that the vibrator is not the key to penetration of the tool, but may be provided merely as a secondary means of achieving penetration into non-cohesive soils. The vibrator may also be used to assist with the extractions of the hole-forming tool.
Alternatively or in addition, the hole-forming tool may be rotated about its longitudinal axis without concrete or grout being pumped, while being pushed into the ground in a substantially non-percussive manner. The rotation may be continuous rotation in a given direction, or may be back and forth rotation as described with reference to the first aspect of the present invention. This rotational movement, when combined with a substantially non-percussive pushing force, allows the hole-forming tool to penetrate the layer or layers of sand or granular material. The rotational movement of the hole-forming tool results in the displacement of soil by the blade-like attachments. As a result of this displacement, ground pressure around the base of the hole-forming tool is lowered, thereby allowing material under the base of the hole-forming tool to move upwards, thereby causing the ultimate end bearing to fail at a much reduced loading.
Once at the required depth the hole-forming tool, or at least the section of the hole-forming tool on which the at least one blade-like attachment is mounted, is rotated in such a way that the at least one blade-like attachment displaces soil at the base of the hole-forming tool, advantageously aided by the pressure of the concrete or grout pumped to the base of the hole-forming tool. As the soil is displaced, it is concomitantly replaced by concrete or grout which may be pumped at positive pressure, for example up to 4 bar or even higher, through the body of the hole-forming tool and emerge from at least one aperture provided behind the at least one blade-like attachment or from at least one aperture provided at or near the tip of the tool, thereby forming a subterranean bulb of concrete or grout: of predictable shape. It is to be noted that as the hole-forming tool is rotated, soil in front of each blade-like attachment is compressed, thereby increasing the local soil pressure relative to the surrounding soil pressure. Correspondingly, soil behind each blade-like attachment is allowed to expand, thereby decreasing the local soil pressure relative to the surrounding soil pressure. Alternatively, in embodiments where the body of the hole-forming tool is solid, concrete or grout may be pumped through a separate feed pipe or pipes. Once the bulb has been formed, the hole-forming tool is extracted, preferably without rotation and preferably in the same orientation as that used during insertion so as to keep resistance to extraction low. In some embodiments, the at least one blade-like attachment may be arranged in a helical configuration about the body of the hole-forming tool. This means that the hole-forming tool may be withdrawn with rotation, provided that the rate of withdrawal is controlled as a function of the pitch of the helix and/or the change of depth and/or the rate of rotation. During extraction, further concrete or grout is delivered so as to form a continuous shaft up to a predetermined level. The delivery of concrete or grout during extraction must be sufficient to ensure that at least a predetermined minimum cross-section of shaft is cast. It is to be noted that with displacement piling tools such as the hole-forming tool hereinbefore described, the soil surrounding the tool will tend to plug any escape path for concrete or grout; consequently, the positive pressure of the concrete or grout can generally be maintained. If the tool is rotated while the concrete or grout pressure is maintained generally constant, then concrete or grout is absorbed into the area surrounding the fins and the original soil is displaced until eventually, after several rotations, substantially all of the original soil is successfully displaced and replaced with concrete or grout. This is readily signalled by a significant drop in the torque required to rotate the tool.
Advantageously, the volume of concrete or grout being pumped during formation of the base and shaft of the cast-in-situ pile is monitored, for example by way of an electromagnetic flowmeter, and controlled by electronic computer means. The electronic computer means may also be adapted to monitor and control the insertion and/or rotation and/or extraction of the hole-forming tool. For example, given the area of the at least one blade-like attachment and the rate of rotation, it is possible to program the electronic computer means to calculate the rate at which the soil is displaced, and consequently to calculate the rate at which concrete or grout should be pumped so as to help ensure that the resulting load-bearing pile or column is structurally sound. The electronic computer means is advantageously adapted to control the rate at which concrete or grout is pumped and/or the rate of rotation of the hole-forming tool in accordance with these calculations. The electronic computer means may additionally be adapted to monitor and control withdrawal of the hole-forming tool and the flow of concrete or grout during extraction so as to ensure that sufficient concrete or grout is supplied.
By displacing the soil at the base of the hole-forming tool and concomitantly injecting concrete or grout, the base of a cast-in-situ pile with enhanced bearing capacity is formed. The enhanced bearing capacity is a result of the increased diameter of the base of the pile relative to the shaft of the pile. The diameter of the base may be several times the diameter of the shaft.
The number of blade-like attachments mounted on the hole-forming tool is preferably two, for reasons of symmetry, but other numbers of attachments may be just as effective. The minimum number of rotations required to achieve the necessary soil displacement is 1/(number of attachments), provided that the attachments are equiangularly spaced about the circumference of the hole-forming tool, although depending on ground conditions, a greater number of rotations may be required so as to ensure that the soil is fully displaced. In some embodiments, the hole-forming tool is rotated first in one direction, and then in the other direction. The rate of rotation is advantageously controlled so that the volume of soil displaced is less than, or at least equal to, the volume of concrete concomitantly supplied.
In a particularly preferred embodiment, the volume of soil displaced is between 50 and 500 litres (0.05 to 0.5 m3), which is relatively efficient in terms of material usage. Additional rotations and additional concrete or grout delivery may also be used in order to increase the size of the bulb, but this may not be of great advantage since the final shape of the bulb may no longer be as predictable and the effective area of the bulb may not increase.
Conventional drive means, such as a motor or even manual means, may be employed to rotate the hole-forming tool. Alternatively, one or more hydraulic rams may be used to rotate the hole-forming tool in a step-wise manner. The one or more hydraulic rams are adapted to engage with arms projecting axially from the body of the hole-forming tool, thereby providing step-wise rotation with a force great enough to ensure that the blade-like attachments displace the soil surrounding the lower end of the hole-forming tool. In a hole-forming tool with two diametrically opposed blade-like attachments, rotation through at least 180xc2x0 is required, whereas in a hole-forming tool with four equally-spaced blade-like e attachments, rotation need only be through at least 90xc2x0.
The blade-like attachments are preferably sized and shaped so as not to deform excessively upon rotation. The blade-like attachments may be solid, or they may be provided with holes or apertures which enable the concrete or grout to be mixed in-situ with soil materials. Alternatively, each blade-like attachment may comprise several protruding elements separated by slits. The precise dimensions and shapes of the blade-like attachments may be selected in accordance with the ground conditions. In some embodiments, it is envisaged that the blade-like attachments may be disposed at an angle to the longitudinal axis of the hole-forming tool, thereby tending to displace soil upwards or downwards upon rotation.
The enlarged base-forming technique described above may be adapted so as to provide an extended enlarged base. This may be done by continuing to rotate the tool while extracting it slowly through the founding stratum and concomitantly supplying concrete or grout. Once the top of the founding stratum is reached, rotation of the tool is stopped, and withdrawal from the ground completed in the normal way. This results in a pile having an elongate base of wider diameter than the main body of the pile, which may have enhanced skin friction as well as enhanced end bearing capacity.
It is also possible for the enlarged base formation technique to be repeated at several depths within the founding stratum, or even to continue rotation while extracting the tool at a rate which allows a continuous spiral base enlargement to be produced.
A further possibility is to apply a degree of back and forth rotation during extraction of the tool. In this way, a pile having a central body is provided with xe2x80x9cwingsxe2x80x9d defined by the width of the blade-like attachments, which may have enhanced skin effects due to the resulting pile having a greater circumferential surface area than that of a simple cylinder.
The cast-in-situ pile may be provided with an enlarged head at or below the surface of the ground by way of additional rotation of the tool and concomitant additional concrete or grout delivery once the hole-forming tool has been withdrawn to a predetermined level. This enlarged head nay be provided just below the ground surface or at greater depths if a reduction of ground level is expected. In some applications, the enlarged head may be formed in a carpet of gravel or other appropriate granular material which has been provided at the surface level.
Once the pile has been cast, a reinforcement may be inserted. This may take the form of a single or multiple steel bar arrangement which is pushed to a predetermined depth into the wet concrete or grout before this has set.